Generally, a direct sequence code division multiple access (DS-CDMA) is one of multi-user communication methods, in which several users can communicate simultaneously by using direct-sequence spread-spectrum(DSSS) techniques in which user data are transmitted after multiplied by an unique code allocated to each user. The chip rate of the code is tens or hundreds times higher than transmit data rate.
Several chip modulation methods for DS-CDMA systems have been proposed for cellular, personal communications services and wireless local loop applications.
Important requirements for a portable terminal in wireless applications with mobility are low cost, low power consumption, small size and light-weight. To meet the requirements of low power consumption, an efficient power amplifier might be used. From the point of view of service providers, spectral efficiency is also an important requirement. However, usually a spectrally efficient signal has large envelope variation. The large envelope variation of the transmitted signal results in increasing the adjacent channel interference and degrading the system performance due to spectral regeneration at the output of a nonlinear power amplifier.
Both non-coherent M-ary orthogonal signaling for data modulation with OQPSK chip modulation EIA/TIA/IS-95, "Mobile station-base station compatibility standard for dual-mode wideband spread spectrum cellular system," July 1993!, and coherent QPSK data modulation with the QPSK data modulation with the QPSK chip modulation Proposed Wideband CDMA PCS Standard, October 1994, OKI Co.!, have been proposed for a DS-CDMA portable terminal. Also coherent QPSK data modulation with the QPSK chip modulation has been proposed for a for a base station of a DS-CDMA cellular telephone system System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System, Jun. 25 1990, U.S. application Ser. No. 543,496, Issue No. 5,103,456!.
In the conventional non-coherent M-ary orthogonal signaling for data modulation with OQPSK chip modulation, log.sub.2 M bits of user data are mapped into one element of a set of M orthogonal binary sequences. Two different PN codes, called inphase and quadrature PN codes, allocated to the user are multiplied by the selected binary sequence, respectively, and input to inphase and quadrature data input port of the conventional OQPSK modulator, respectively. Using the non-coherent OQPSK PN modulation for a portable terminal, due to a delay of a half PN chip duration in the quadrature component of a transmitted signal, there is some self interference between the inphase and the quadrature components, although it is typically much smaller than the multiple access interference from the other users.
In a coherent direct sequence spread spectrum communication system, a pilot signal should be sent with the data signal to inform the other party on the phase information of the carrier modulated by the data signal. Commonly, the pilot signal is also a direct sequence spread spectrum signal in which a sequence with all zero's in logical value is spread by PN codes. The correlation coefficients of PN codes used in spreading data and those in spreading pilot symbols are zero or very low values such that the other party can distinguish between the pilot signal and the user data signal.
In the conventional coherent QPSK data modulation with the QPSK chip modulation, there are three channels; pilot, data and signaling channels. In this method, the pilot symbols, signaling symbols and user data bits, are spread by a same PN code, respectively. After spreading, they are covered by different Walsh sequences, where different Walsh sequences are orthogonal to each other, to be distinguished among them at the other party. The sum of pilot and data signals and the sum of signaling and data signals are input to the inphase data and quadrature data input ports of the conventional QPSK modulator, respectively. Due to the sum of two signals the envelope variation of transmitted signal becomes very high.
Using the coherent QPSK data modulation with QPSK chip modulation for a portable terminal, the sum of the pilot and the data channels exhibits a high envelope variation and a large phase change. Even though the envelope variation of transmitted signal is a minor problem in a base station since in the base station the power consumption is not critical, the reduction of envelope variation of transmitted signal might be helpful for improvement of whole system performance since the portable terminal might use a limiter at the receiver to reduce the terminal cost by eliminating the emphasis of the receiver AGC chain J. Boccuzzi, "Performance Evaluation of Non-linear Transmit Power Amplifier," IEEE Transactions on Vehicular technology, No. 2, pp. 220-228, May 1995!.
It is an object of this invention to provide a special case of QPSK PN modulation, called Walsh-QPSK, which can exhibit lower envelope variation and smaller phase changes than the conventional PN modulation schemes.